I. Field of the Invention
This invention relates generally to implantable medical tissue stimulating apparatus, and more particularly to a programmable implantable medical device having an electronic therapy delivery circuit and a telemetry system for allowing duplex communication between the implanted therapy delivery circuit and an external programmer where the implanted telemetry transmitter and receiver circuits are effectively isolated from the output of the therapy delivery circuit to thereby prevent damage to the telemetry transmitter and receiver circuits.
II. Discussion of the Prior Art
Implantable medical stimulating devices, such as Automatic Implantable Cardiac Defibrillators (AICD), typically comprise a hermetically sealed housing or enclosure made of titanium or other suitable metal that is body compatible and moisture impervious. Contained within the housing is electronic circuitry for monitoring cardiac activity and a therapy delivery circuit for producing tissue stimulating pulses under control of a programmable microprocessor. The enclosure of these implantable medical devices will also typically include a telemetry transmitting circuit and a telemetry receiving circuit, allowing two-way communication with an external programmer.
Affixed to the exterior of the housing of these prior art implantable devices is a molded plastic header having one or more lead receiving bores formed longitudinally therein. The bores are provided with electrical contacts for mating with corresponding contacts on the proximal end of an elongated, flexible therapy delivery lead of the type having tissue contacting electrodes at the distal end thereof. The electrodes are connected to the contacts on the proximal end of the lead by means of conductors extending through the insulative lead body, all as is well known in the art.
The header may also support either an antenna or an electromagnetic coil as a transducer used to transmit and receive data being telemetered to and from the implantable device. To establish a connection between the sensing circuitry, the therapy delivery circuitry and the telemetry transmitter and receiver within the housing and the lead contacts and the transducer on the header, hermetically sealed feed-through pins are typically provided that extend through insulating seals mounted in the enclosure beneath the header to contact points within the header to which the lead barrel contacts and the telemetry antenna/coil connect. Typical feed-through devices are described in U.S. Pat. No. 5,333,095. Because of their cost and complexity, the number used in a device is to be minimized.
Biomedical devices, particularly implantable medical devices, such as cardiac defibrillators, can impress very large shocking energies on the patient to thereby resynchronize heart function. In some defibrillator designs, the titanium enclosure or housing is used in combination with a lead electrode as an opposing electrode in applying this high-energy stimulation. The term xe2x80x9cHot Canxe2x80x9d has been used to describe this lead electrode-to-case stimulation approach. Because the implantable device""s external casing is used as an electrode, much of the circuitry enclosed thereby cannot use the case as a voltage reference point, such as a ground reference point, like many other implantable devices. Failure to adequately isolate the telemetry circuitry from Hot Can shocks can cause electrical damage to the telemetry transmitter and receiver and this can adversely impact the ability of the device to also apply shocking therapy when needed. Such isolation is also necessary to insure that other devices, such as pacemakers, located in proximity to a defibrillator are not damaged by high-energy shocking potentials.
Most prior art telemetry systems used with implantable medical devices utilize magnetic or electromagnetic fields to perform the transfer of data from the implantable device to an external programming device for clinician or patient use. Magnetic field telemetry uses an inductive coil disposed in a wand and placed over the site of the implantable device to magnetically couple to a receiving coil either within the header or within the enclosure of the implanted unit. In the case of electromagnetic field telemetry, an antenna in the external wand transmits RF energy to a corresponding antenna disposed in or on the header of the implanted unit.
As is explained in the deCoriolis et al. U.S. Pat. No. 5,342,408, it is advantageous to have the coil/antenna of the implantable device disposed outside of the metal housing to avoid the shielding losses that occur as the frequency of the RF telemetry carrier signal increases. The involvement of the housing in these higher frequency forms of telemetry becomes increasingly critical for two reasons. The first reason is that the case becomes a ground plane or shield to shape or control magnetic and electromagnetic field propagation. The second reason is that the case becomes a convenient circuit tie point, thereby avoiding the need for an additional costly feed-through assembly in the device. As those skilled in the art appreciate, feed-throughs are structures to be minimized in implantable devices since they become entry points for electro-magnetic interference and need to be designed with high electrical and mechanical precision to limit such interference.
A need therefore exists for a way to permit Hot Can therapy delivery in an implantable medical device that also incorporates telemetry circuits such that the telemetry circuits are effectively isolated from high shocking potentials while yet minimizing the number of feed-throughs employed for conductively connecting the therapy delivery circuit to the lead and the transmitting/receiving electronics to a coil/antenna disposed in or on the device""s header.
The foregoing need is met in accordance with the present invention by providing an implantable medical device having a moisture impervious, body-compatible, metal housing containing a therapy delivery circuit, a telemetry transmitter and a telemetry receiver. Affixed to the metal housing of the device is an electrically non-conductive header which supports a magnetic or electromagnetic field energized telemetry coil/antenna. At least one hermetically sealed feed-through pin extends from the header where it is connected to the coil/antenna into the metal housing. Disposed within the housing is an isolation transformer having a primary winding and a first secondary winding where at least one terminal of the primary winding is coupled to the feed-through pin and the first secondary winding is coupled in circuit with at least one of the telemetry transmitter and telemetry receiver to permit signal flow between the transmitter or receiver and the telemetry coil/antenna. The isolation transformer serves to electrically isolate the telemetry transmitter or receiver from voltages impressed on the housing by the therapy delivery circuit.
The isolation transformer may have a further secondary winding where the first secondary winding connects to the telemetry transmitter and the other secondary winding connects to the telemetry receiver.
In another alternative embodiment, a two electrode therapy lead having an integral telemetry antenna connects to the device header for twin feed-through connection to an internal isolation transformer primary winding. In this arrangement, the therapy delivery circuit connects to these same two feed-through pins but signals from the therapy delivery circuit are attenuated by a high-pass filter before reaching the isolation transformer primary winding.